1. Field of the Invention
The present invention relates to a mobile communication system. More particularly, the present invention relates to an apparatus and method for adapting different weights according to change in a Transmit (Tx) Diversity scheme when a Node B transmits data using a Tx Diversity scheme.
2. Description of the Related Art
As mobile telecommunication systems have been developing and the amount of data transmitted on such systems has been increasing, a third generation mobile telecommunication system providing data at a higher speed has recently been developed. As for the third generation mobile telecommunication system, a Wideband-Code Division Multiple Access (W-CDMA) format which is asynchronous has been widely used in Europe as a wireless access standard, and a Code Division Multiple Access-2000 (CDMA-2000) format which is synchronous has been widely used in North America as a wireless access standard. Typically, the mobile telecommunication system enables a plurality of User Equipments (UE's) to communicate via one Node B. However, phase distortion of a reception signal occurs in the mobile telecommunication system resulting in a fading phenomenon during transmission of high-speed data. The fading phenomenon causes an amplitude of a reception signal to be reduced from several tens of dBs to a few dBs. Therefore, if the distorted phase of the reception signal is not compensated for during a data demodulation process, an undesired information error occurs in the transmission data from a transmission end such that a Quality Of Service (QoS) of the mobile telecommunication system deteriorates. In order to transmit high-speed data without QoS deterioration, the fading phenomenon must be addressed. A variety of diversity schemes have been widely used to overcome the fading phenomenon.
Typically, a CDMA scheme uses a rake receiver for performing diversity reception using a delay spread of a channel signal. A general rake receiver uses a Receive (Rx) Diversity scheme for receiving a multi-path signal, and each finger of the rake receiver is assigned one signal path such that it performs a demodulation process. However, the above rake receiver based on a diversity scheme using the delay spread is inactive when a value of the delay spread is lower than a prescribed value. Also, a Time Diversity scheme using an interleaving operation and a coding operation is typically used for a Doppler spread channel. However, it is difficult to use the Time Diversity scheme in a low-speed Doppler spread channel.
Therefore, in order to solve the fading phenomenon, a Space Diversity scheme has been used for both a channel having a low delay spread, such as an indoor channel, and a channel having a low-speed Doppler spread, such as a walker channel. The Space Diversity scheme is indicative of a diversity scheme for use with at least two transmission and reception antennas. Specifically, if the magnitude of a signal transmitted via one antenna is reduced by the fading phenomenon, the Space Diversity scheme demodulates a transmission signal by receiving signals transmitted via the remaining antennas. The Space Diversity scheme is classified into a Receive Antenna Diversity scheme using a reception antenna and a Transmit Antenna Diversity scheme using a transmission antenna. However, because the Receive Antenna Diversity scheme is used for UEs, it is difficult to install a plurality of antennas for each UE due to the size and cost of each UE. Therefore, the Transmit Antenna Diversity scheme for installing many antennas to a Node B is preferably used.
The Transmit Diversity scheme controls a transmission end to transmit signals over multiple antennas, and controls a reception end to receive, demodulate, and combine output signals of individual antennas to overcome fading channels. The W-CDMA implements the Transmit Diversity scheme by adapting two antennas to the Node B according to specified communication standards.
The Transmit Antenna Diversity scheme uses a specific algorithm for receiving a downlink signal to obtain a diversity gain, and is classified into an open loop transmit diversity and a closed loop transmit diversity. The open loop transmit diversity is classified into a Time-Switched Transmit Diversity (TSTD), and a Space-Time Transmit Diversity (STTD), and the closed loop transmit diversity includes a Transmit Antenna Array (TxAA) scheme.
For the open loop transmit diversity, if a Node B encodes information bits and transmits them via a plurality of diversity antennas, the UE receives signals transmitted from the Node B and decodes the received signals such that a diversity gain is obtained.
For the closed loop transmit diversity, if the UE estimates and calculates channel environments through which signals transmitted via transmission antennas of a Node B will travel in the future, calculates weights of the antennas of the Node B on the basis of the calculated estimation values in order to obtain a maximal power value of a reception signal, and finally transmits the weights to the Node B via an uplink, the Node B receives the weights from the UE and applies each of the weights to each antenna, thereby adapting received weights to individual antennas. In this case, the Node B transmits a pilot signal for every antenna to measure a channel of the UE, and thereby the UE measures a channel using the pilot signal for every antenna and generates an optimal weight based on the measured channel information.
The closed loop transmit diversity method will hereinafter be described with reference to FIG. 1. FIG. 1 is a block diagram illustrating a TxAA-scheme transmitter indicative of a representative technique of the closed loop transmit diversity scheme.
A Dedicated Physical Control CHannel (DPCCH) for transmitting a plurality of control signals and a Dedicated Physical Data CHannel (DPDCH) for transmitting information signals are multiplexed to configure a Dedicated Physical CHannel (DPCH) 102. In this case, the DPCCH and the DPDCH are typically time-multiplexed in a downlink direction.
The multiplier 104 scrambles the DPCH signal using a scrambling code. The scrambled DPCH signal is multiplied by a predetermined weight W1 in the multiplier 106, and is multiplied by a predetermined weight W2 in the multiplier 108, such that the closed loop transmit diversity is applied to the scrambled DPCH signal. Separate calculation signals are transmitted over a first antenna 114 and a second antenna 116.
The closed loop transmit diversity scheme is classified into a first mode scheme and a second mode scheme according to weight use methods. Specifically, the first mode scheme (Mode 1) determines a weight by considering only a phase difference between signals received over the two antennas 114 and 116, and the second mode scheme (Mode 2) determines a weight by considering the phase difference and the magnitude difference between signals received over the two antennas 114 and 116.
Signals transmitted to the first and second antennas are multiplexed with unique pilot signals CPICH1 and CPICH2 assigned to every antenna in individual multiplexers 110 and 112, respectively, such that the state information of channel signals transmitted over individual antennas can be measured.
The Mobile Station (MS) receives signals from individual antennas, measures channel states of individual antennas using the pilot signals, and multiplies weights by pilot signals of two antennas, such that it determines a specific weight for maximizing a power level of a reception signal indicative of the sum of the multiplied results in association with individual antennas. Information of the weight is defined as predetermined sets. If the weight from among three available weights is adapted to a reception end, the reception end determines one weight for maximizing a necessary reception power. If information of the determined weight is transmitted over the feedback information (FBI) field of an uplink DPCCH message, the FBI message determination unit 118 analyzes the feedback information received from the reception end. The weight generator 120 generates weights W1 and W2 for every antenna, and multiplies the weights W1 and W2 by a DPCH signal to be transmitted.
The uplink DPCCH configuration and the FBI configuration will hereinafter be described with reference to FIGS. 2 and 3. FIG. 2 is a view illustrating a configuration of the uplink DPCCH. One frame of the uplink DPCCH comprises 15 slots, and each slot includes a pilot symbol, a Transmit Format Combination Indicator (TFCI) bit, a feedback information (FBI) symbol, and a downlink Transmit Power Control Commander (TPC). The pilot symbol is used as a channel estimation signal when data transmitted from the MS to the Node B is demodulated. The TFCI bit indicates which one of the Transport Format Combinations (TFCs) is applied to a downlink channel transmitted during a current transmission frame. The FBI symbol transmits feedback information according to the used Transmit Diversity scheme, and the TPC symbol is a symbol for controlling Downlink Channel Transmit Power. The uplink DPCCH is spread using an orthogonal code, and is transmitted to a destination. In this case, a Spreading Factor (SF)is fixed at a specific number, for example, 256.
FIG. 3 illustrates a configuration of an FBI symbol included in the uplink DPCCH. The FBI symbol is classified into an S field and a D field. The S field is indicative of a field for the STTD signal, and the D field is indicative of a field for the closed loop transmit diversity. Therefore, the MS transmits weight information over the D field of the uplink DPCCH's FBI symbol. The D field includes a maximum of 1 bit for every slot.
A channel for use in the W-CDMA communication system is mainly classified into a Physical Channel, a Transport Channel, and a Logical Channel according to signal processing of hierarchical information. The physical channel for spatially transmitting data is classified into a downlink physical channel and an uplink physical channel according to information transmission directions.
The Transmit Diversity scheme for use in individual physical channels is shown in the following Table 1:
TABLE 1Open loop TransmitClosed loop TransmitDiversityDiversityPhysical Channel TypeTSTDSTTDMode 1Mode 2P-CCPCHX◯XXSCH◯XXXS-CCPCHX◯XXDPCHX◯◯◯PICHX◯XXPDSCHX◯◯◯HS-PDSCHX◯◯XHS-SCCHX◯◯XAICHX◯XXCSICHX◯XXAP-AICHX◯XXCD/CA-ICHX◯XXDL-DPCCH for CPCHX◯◯◯
In this case, the principles for adapting the Transmit Diversity scheme to the physical channels will hereinafter be described.
1) According to the first principle, the STTD scheme and the closed loop transmit diversity scheme cannot be applied to the same physical channel at the same time.
2) According to the second principle, when adapting the Transmit Diversity scheme to one of downlinks, the Transmit Diversity scheme must be applied to the P-CCPCH and the SCH.
3) According to the third principle, a PDSCH and its associated DPCH must use the same Transmit Diversity scheme at all times.
4) According to the fourth principle, the HS-SCCH, the HS-PDSCH, and their associated DPCH must use the same Transmit Diversity scheme.
The UMTS system based on the W-CDMA scheme must combine DPCH signals transmitted from several Node Bs at a soft handover time. The Node Bs for transmitting the DPCH signals to be received at the same time must satisfy the following regulations associated with Transmit Diversity scheme usage according to prescribed standards.
1) According to the first regulation, the Node Bs use one Transmit Diversity scheme when transmitting a DPCH signal to a desired UE. Specifically, the Node Bs avoid duplication of the open loop transmit diversity scheme and the closed loop transmit diversity scheme.
2) According to the second regulation, if all the Node Bs currently providing UEs with necessary services do not use the Transmit Diversity scheme, a recently added Node B for DPCH transmission is not affected by conventional Node Bs in association with specific information indicative of a used or unused Transmit Diversity scheme.
3) According to the third regulation, if one or more Node Bs from among a plurality of Node Bs currently providing UEs with necessary services transmit the DPCH using the open loop transmit diversity scheme, the recently added Node B can transmit the DPCH using the open loop transmit diversity scheme or can also transmit the DPCH without using the transmit diversity scheme.
4) According to the fourth regulation, if one or more Node Bs from among a plurality of Node Bs currently providing UEs with necessary services transmit the DPCH using the closed loop transmit diversity Mode1 scheme, the recently added Node B can transmit the DPCH using the closed loop transmit diversity Mode1 scheme or can also transmit the DPCH without using the transmit diversity scheme.
5) According to the fifth regulation, if one or more Node Bs from among a plurality of Node Bs currently providing UEs with necessary services transmit the DPCH using the closed loop transmit diversity Mode2 scheme, the recently added Node B can transmit the DPCH using the closed loop transmit diversity Mode2 scheme or can also transmit the DPCH without using the transmit diversity scheme.
Upon receiving downlink DPCH signals from more than two Node Bs at a soft handover time, individual Node Bs are not affected by specific information indicating whether the Transmit Diversity scheme is adapted to the Node Bs. When using the Transmit Diversity scheme, the closed loop transmit diversity scheme and the open loop transmit diversity scheme cannot be used at the same time. In the case of using the closed loop transmit diversity scheme, the aforementioned Mode1 and Mode2 schemes cannot be used at the same time. A plurality of Node Bs for providing UEs with necessary services must select one scheme from among a plurality of Transmit Diversity schemes, and must use only the selected scheme.
A method for controlling the MS to generate weights will hereinafter be described. The MS performs channel estimation using the CPICH, and this channel estimation value configured in the form of a matrix H can be represented by the following equation 1:H=[h1, h2]
With reference to Equation 1, h1 and h2 are channel estimation vectors for two transmission antennas, respectively. The MS selects weights w1 and w2 capable of maximizing the result value of the following equation 2 using the channel estimation vectors.P=wHHHHw
With reference to Equation 2, w is [w1, w2]T, and P is indicative of reception power when weight information is adapted to the first and second antennas in current channel environments. The MS searches for a weight for maximizing the reception power, and transmits the weight to the Node B.
FIG. 4 is a configuration of a Feedback Signal Message (FSM) transmitted from the MS to the Node B to establish the closed loop transmit diversity scheme. The FSM configuration is transmitted over the D field contained in the FBI field shown in FIG. 3. The FSM is classified into an FSMph field indicative of phase information and an FSMpo field indicative of magnitude information.
If the MS performs a handover function as described above, a Transmit Diversity scheme for use in the former Node B before performing the handover function may be different from the other Transmit Diversity scheme for use in the latter Node B after performing the handover function. Provided that the Transmit Diversity scheme of the former Node B is equal to that of the latter Node B, the MS need not change the Transmit Diversity scheme. Otherwise, provided that the Transmit Diversity scheme of the former Node B is different from that of the latter Node B, the MS must change the Transmit Diversity scheme according to handover situations.
If the MS changes the Transmit Diversity scheme, and particularly, if there arises a variation in mode of the closed loop transmit diversity scheme, the MS must establish an initial weight at a time of changing the mode to another mode. The Node B must receive weight information capable of maximizing reception power in the MS, and transmit data using the received weight information. However, as shown in FIG. 4, the weight information comprises 2 bits when the closed loop transmit diversity scheme is set to Mode1, comprises 4 bits when the Transmit Diversity scheme is set to Mode2, and one bit is transmitted over one slot. The Node Bs take a predetermined time to receive all of the weight information transmitted from the MS in such a way that they can receive appropriate weights from the MS. Therefore, the Node B must transmit data to be transmitted prior to the predetermined time using a predetermined weight determined between the Node B and the MS. However current channel environments are not reflected in the aforementioned case, such that the weight for maximizing the reception power is not applied to the aforementioned case, and an optimum weight is not applied to signals generated until applying a weight where a channel situation is reflected, resulting in deterioration of system performance. In conclusion, a method must be developed for solving the aforementioned problems during a handover.